Hedge-fund manager Bill Ackman is short the nutritional-supplement seller Herbalife, according to someone familiar with the matter, and has told people he considers the business to be little more than a "pyramid scheme."

Ackman, founder of the $11 billion hedge-fund company Pershing Square Capital Management, put on the bearish Herbalife position about seven or eight months ago, according to the person familiar with the matter, and has spent about a year researching the company.

The fund manager has told people that Herbalife—best known for its weight-loss shakes—is the most compelling short case for a stock he has "ever seen," this person added. A "short" is a bet that a stock's price will decline.

A pyramid scheme is an illegal investment scam where contributions from new investors are used to pay off the original investors.

Ackman will provide more details on his short thesis as part of a two-hour presentation at the Sohn Conference Foundation investment gathering Thursday morning in midtown Manhattan, according to people familiar with the agenda.